1. Field of the Invention
The invention is a three-stage process for removing acetylene and ethylene from hydrogen chloride from pyrolysis of 1,2-dichloroethane.
In large scale industrial processes for the production of vinyl chloride by pyrolysis of 1,2-dichloroethane, large quantities of hydrogen chloride are produced. The hydrogen chloride is contaminated with 0.05 to 0.5 mol % of acetylene and 0.005 to 0.05 mol % of ethylene. After removal of the acetylene by selective hydrogenation over platinum metal catalysts or platinum metal oxide catalysts to convert the acetylene to predominantly ethylene, the hydrogen chloride is usually employed for the oxychlorination of ethylene to give 1,2-dichloroethane (DE-A 2,353,437, DE-B 1,568,679=GB A 1,090,499 and DE-A 3,043,442=U.S.A 4,388,278).
With increasing interest in silicon chemistry, the hydrogen chloride demand is also increasing (preparation of trichlorosilane as the starting material for the chemistry of highly pure silicon, or preparation of methyl chloride for reaction with silicon as a starting material for preparation of silicon containing compositions). Even though various sources of hydrogen chloride are known, hydrogen chloride from the pyrolysis of 1,2-dichloroethane, for the production of vinyl chloride represents a particularly economical and always available raw material source for hydrogen chloride. A disadvantage of this hydrogen chloride is that the impurities resulting from the process such as acetylene or its partial hydrogenation product ethylene, as well as hydrochlorination products and decomposition products which are formed in the acetylene removal process, remain in the hydrogen chloride (vinyl chloride, ethyl chloride, ethane, (un)-saturated C.sub.4 -hydrocarbons) and cause extremely serious interference, even in the concentration range of ppm by volume, in the synthesis of the intermediates for the preparation of silicon containing compositions.
2. Statement of Related Art
High purity hydrogen chloride can be obtained by combustion of chlorine in a stream of hydrogen. This synthetic hydrogen chloride is very expensive and generally contains an excess of hydrogen.
A further possibility for producing high purity hydrogen chloride is based on purifying contaminated hydrogen chloride from pyrolysis of 1,2-dichloroethane by absorption in azeotropic hydrochloric acid, while blowing with air as the stripping gas. The hydrogen chloride is removed by subsequent thermal desorption from concentrated hydrochloric acid. However, this process is very expensive and involves enormous corrosion problems, and overall is generally uneconomical.
U.S. Pat. No. 3,923,963 discloses a process for purifying hydrogen chloride in which unsaturated hydrocarbon compounds or chlorohydrocarbon compounds are removed by reacting these compounds with at least stoichiometric quantities of chlorine in an activated carbon bed at not less than 80.degree. C. and under pressure from 45 to 11.5 bar. The content of chlorinated by-products and excess chlorine is reduced in each case to less than 1 ppm by adsorption on the activated carbon. The disadvantage of this process is that the activated carbon must be regenerated by desorption. The activity of the activated carbon decreases rapidly. Because of the residual chlorine content, the hydrogen chloride treated in this way cannot be used to prepare silicon compounds which require pure hydrogen chloride.
In DE-A 3,508,371, a two step process for purification of pyrolysis hydrogen chloride, after acetylene removal by hydrogenolysis, is disclosed. The ethylene is catalytically hydrogenated to ethane in the first stage and in the second stage, the hydrogen chloride is freed of hydrochlorination products and decomposition products by pressure rectification at low temperatures. The disadvantage of this purification process is that the hydrogen chloride purified in this way contains ethane and excess hydrogen as an inert gas sc that environmental problems and condensation problems arise.
It is therefore an object of the present invention to provide an economical process for purifying hydrogen chloride from pyrolysis of 1,2-dichloroethane, which process is satisfactory with respect to the purity and inert gas content.